Velocity Potential, Stream function, Divergence, Vorticity
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   The velocity potential, stream function, divergence, and vorticity on
pressure surfaces were calculated by the following procedure:

1)  Winds on sigma surfaces were calculated on a 144x73 lat-long grid.
    The spectral transform goes directly to the lat-long grid
    (exact process).

2)  Winds were vertically interpolated to pressure surface using a
    linear (log-p) interpolation (inexact process).

3)  The gridded data were written in GRIB (inexact process).

4)  Winds were converted to spectral coefficients with an implicit 
    T36 spectral filtering (inexact process).

5)  Spectral winds were converted to velocity potential, stream function,
    divergence and vorticity (exact process).

6)  Spectral quantities were converted to gridded quantities (exact
    process) and put in to GRIB format (inexact process).


exact process = no loss of precision except for numerical round-off
inexact process = loss of precision by design or by an approximation.

Note: step 4 could be made into an exact process if the data were on a 
Gaussian grid rather than lat-long grid.


   The above procedure used is much more accurate than computing the
quantities from the gridded winds using a low-order finite difference
scheme.